GASTEROPODA. 401 



e.g. Heteropods. There are never more than two gills of the 

 ctenidium type. The shell is usually in one piece ; but it is 

 sometimes rudimentary or absent. The foot usually contains 

 a mucus gland, and tends to be divided into three regions the 

 pro-, meso-, and metapodium. There is a single reproductive 

 organ and genital duct. 



Branch A. STREPTONEURA. 



In the torsion of the body one limb of the visceral loop crosses the 

 other in a figure 8. 



Order i. ZYGOBRANCHIATA. 



The atrophy of the primitively left-side gills and nephridia is not 

 carried out, or only partially, e.g. Haliotis (ear-shell) ; frissurella (key- 

 hole limpet) ; Patella (limpet). 



Order 2. AZYGOBRANCHIATA. 



The originally left gill and the originally left nephridium have been 

 lost. Heart with single auricle, one gill, one nephridium ; operculum 

 present. 



Periwinkle (Littorina), buckie (Bitccinum, Fig. 194), Dog-whelk 

 (Purpura), lanthina, and the majority of the marine Gasteropods 

 with coiled shells, together with some fresh -water forms. The 

 pelagic Heteropods are also included here : Atlanta, shell well 

 developed ; CarinaHa, with small shell ; Pterotrachea, with no 

 shell. 



Branch B. EUTHYNEURA. 



The visceral loop does not share in the torsion of the visceral hump. 



Order 3. OPISTHOBRANCHIATA. 



The visceral loop is euthyneural, as in snails ; the single auricle lies 

 behind the ventricle ; the shell and mantle are often absent. 



A. Tectibranchiata. A shell is present, but may be rudimentary ; 

 there is a well-developed mantle fold and a single gill, e.g. 

 Bulla, Aplysia, Dolabella, Umbrella. The Tectibranchiata 

 also include the Pteropoda, the winged snails or sea-butter- 

 flies, which have become much modified for pelagic life. 

 They have a secondarily acquired bilateral symmetry, and 

 swim by two large lateral lobes of the foot. They often 

 swim actively in shoals, and occur in all seas. They afford 

 food for whales, etc., and the shells of some are abundant in 

 the ooze. They include 



(a) Thecosomata, with mantle fold and shell, diet of 

 minute animal or vegetable organisms, closely related 

 to Bulla and its allies. 



Examples. Hyalca, Cymbtilia. 



26 



